In web-fed printing press systems, a web of material (typically paper) is sequentially driven through one or more printing units comprising a plate cylinder and a print cylinder (blanket cylinder). The blanket cylinders sequentially contact &he web and apply ink thereto to form a composite printed image on the web. As the web exits the printing units, the ink is wet and thus subject to smearing. Accordingly, before further processing, e.g., cutting and folding, the web is typically routed through a drying unit to heat the web to evaporate various solvents in the ink and thus dry the image, then to a chill roller unit to cool the web and set the ink.
Typical drying units comprise an enclosed, elongated housing, enclosing a drying chamber including a plurality of upper and lower air nozzle assemblies spaced apart along its length. The traveling web enters the dryer through a narrow entrance slot and is supported by pressurized, heated air from the nozzle assemblies until it exits the dryer through a discharge slot. In this way, the web may travel through the drying unit in a sinusoidal fashion without coming into physical contact with the drying unit; marring of the image before the ink is fully dried is thus avoided.
During steady state operation of a typical high speed press, the web travels at approximately 2,200 feet per minute (approximately 440 inches per second). If the web breaks downstream of the print cylinders, for example in the dryer, the press will continue to operate at normal speed until the break is detected. It is therefore desirable to quickly detect a web break and to immediately stop the press to minimize wasted paper, ink, and production time.
A more compelling reason for rapid shut-down after a web break surrounds the effect of a web break on a blanket cylinder. The circumferential surface of a blanket cylinder comprises a spongy material which transfers ink from its associated plate cylinder to the web. Each blanket cylinder thus imprints that portion of the composite printed image corresponding to a particular plate cylinder. In printing systems which employ a pair of blanket cylinders on the upper and lower surface of a web to simultaneously apply images to both sides of the web, a nip point is established at the contact point between each of the two blanket cylinders and the web. The cushioned circumferential surfaces of the blanket cylinders are configured to compress slightly at the nip point, thereby maintaining a controlled degree of pressure at the nip point. If the web breaks downstream of the blanket cylinder, the web often tends to wrap around the blanket cylinder. With continued operation, the web continues to wrap around the blanket cylinder, causing multiple layers of paper to pass through the nip point, such that the cumulative thickness of paper increases with each successive blanket wrap. This tends to compress the resilient "blanket". As the number of wraps becomes excessive, for example more than three or four wraps, the blanket may become permanently damaged, requiring removal and replacement of the blanket. Accordingly, it is desirable to terminate press operation before irreparable damage is done to the blanket cylinders.
Presently known systems for detecting a web break and terminating press operation in response thereto are unsatisfactory in several regards. For example, presently known web detection systems are disposed either intermediate the printing units and the dryer or after the dryer, and thus typically do not detect a web break which occurs in the dryer until the break point either travels through the dryer or is pulled back through to the entrance of the dryer, resulting in a delay of up to one second or more between the occurrence of the web break and the detection of the web break.
Web break detection systems employing an infrared sensor in conjunction with fiber optic cables to sense the presence of the web within the dryer are known. One example of such a system is the Dryer Web Break Detector, Model 1126 manufactured by the Baldwin Technology Corporation of Stamford, Conn. However, the sensing mechanism tends to become obscured by the various chemicals, solvents, and smoke produced within a dryer environment, resulting in a relatively low degree of reliability of such "in-dryer" web detection systems.
Another known web detection system directs a sonic wave of known frequency at the web surface, and compares the frequency of the reflected wave to the incident wave. If the frequency of the reflected wave is greater or less than the incident wave, or if no reflected wave is sensed, the system indicates a web break.
Another known device comprises a feeler gauge; a spring biased feeler is placed in contact with the web surface. If the web breaks, the feeler pivots toward the plane of the web, triggering generation of a web break signal.
It is also known to dispose a spring loaded switch directly below the web surface and to direct an air stream at the upper surface of the web opposite the switch. As long as the web is intact, the air stream impinges on the web and does not actuate the switch. When the web is broken, the air stream is permitted to actuate the switch, signalling a web break.
To the extent the foregoing systems employ electrical components or moving mechanical components, they are unsatisfactory for use within a dryer, where temperatures may approach 500.degree.-600.degree. F.
A web detection system is needed which overcomes the shortcomings of existing sensing apparatus. Specifically, a web detection system is needed which is capable of sensing the presence or absence of a web within a dryer environment, and which is capable of reliably sensing a web break at high temperatures and in the presence of solvents and dense smoke.